Water-in-oil emulsion explosives are well-known in the art. See, for example, U.S. Pat. Nos. 4,356,044; 4,322,258; 4,141,767; 3,447,978 and 3,161,551. Emulsion explosives are found to have certain advantages over conventional aqueous slurry explosives, which have a continuous aqueous phase, as described in U.S. Pat. No. 4,141,767.
An inherent problem with emulsion explosives, however, is their relative instability, due to the fact that they comprise a thermodynamically unstable dispersion of supercooled solution or melt droplets in an oil-continuous phase. If the emulsion remains stable, these supercooled droplets are prevented from crystallizing or solidifying into a lower energy state. If the emulsion weakens or becomes unstable, however, then crystallization or solidification of the droplets results, and the explosive generally loses some of its sensitivity to detonation and becomes too viscous to handle for certain blasting applications. Moreover, it is common to add solid components to emulsion explosives in the form of glass microspheres for density reduction and prills or particles of oxidizer salt such as porous prilled ammonium nitrate (AN) for increased energy. These solid components, however, tend to destabilize emulsions.
Emulsion explosives commonly are used as a repumpable explosive, i.e., an explosive that is formulated at a remote facility, loaded or pumped into a bulk container and then transported in the container to a blasting site where it then is "repumped" from the container into a borehole. Alternatively, the explosive may be delivered (repumped) into a centrally located storage tank from which it will be further repumped into a vehicle for transportation to a blasting site and then again repumped into the borehole. Thus the emulsion explosive must remain stable even after being subjected to repeated handling or shearing action, which normally also tends to destabilize an emulsion. Additionally, the emulsion--s viscosity must remain low enough to allow for repumping at reasonable pressures and at the low ambient temperatures that may be experienced during colder months. Repeated handling or shearing action also tends to increase the emulsion--s viscosity.
Since a density control agent is required in many instances to reduce the density of an explosive and thereby increase its sensitivity to a required level for detonation, and since hollow microspheres are a preferred form of density control, it is important that the emulsion remain stable and have a low viscosity even when containing solid density control agents. Heretofore, stability has been a significant problem in repumpable emulsion explosives containing solid density control agents. Moreover, certain types of solid density control agents are more destabilizing to emulsions than others. For example, at 20.degree. C. one of the more commonly used emulsifiers, sorbitan monooleate, forms a relatively stable emulsion in the presence of glass microspheres but is less effective in the presence of organic microspheres.
A major advantage of the present invention is that the use of a polypropene or polybutene derivatized emulsifier in combination with organic microspheres as the solid density control agent imparts improved stability and lower viscosity to the emulsion, even after repeated handling or shearing action. Another advantage is that the use of polypropene or polybutene derivatized emulsifiers allows for organic microspheres to be used as a density control agent without adversely affecting the stability of the emulsion. Still another advantage is that the organic microspheres are a reactive fuel, rather than an inert like glass microspheres, and thus contribute to the detonation reaction. The presence of organic microspheres in compositions of the present invention has been found to result generally in higher detonation velocities, higher detonation sensitivities and lower critical diameters.